Previous studies have identified a large population of GABA intrinsic neurons in the monkey primary visual cortex (area 17). These neurons are morhologically and biochemically diverse and appear responsible for intracortical inhibitory processing of visual information. The long term objective of these studies is to determine the contribution of GABA neurons, including those which may contain one or more neuroactive peptides, to the functional circuitry within area 17. Of particular interest is their contribution to the transfer of input from the dorsal lateral geniculate nucleus to the cortex and their response to reductions in synaptic activity. The proposed studies will utilize quantitative and qualitative light and electron microscopic methods to examine immunocytochemically prepared material; double labeling immunocytochemical methods will be used to examine possible neurotransmitter co-existence. By examining the neurotransmitter attributes of neurons in a physiologically well-characterized area of primate cortex, these studies will address the questions of how various types of biochemically distinct cortical neurons may function under normal conditions and how they respond to changes in the levels of their activity. These data may be significant to understanding how populations of GABA and neuropeptide neurons function in certain pathologies, including focal epilepsy and senile dementia of the Alzheimer's type.